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As a plane with an airfoil wing moves forward, the air around it is split above and below the wing. The upper surface of the wing is curved so that the air above increases in speed and decreases in pressure as it travels over the wing. The lower surface of the wing is designed so that the air below the wing remains constant in speed and pressure. Air below the wing moves in a straight rather than curved line.

Areas of high pressure always move toward areas of low pressure. Following this principle, the air below the wing blows up toward the air above. This causes the wing to be pushed upward, and this force is proportional to the speed of the plane. The speed of the plane determines the speed of the air traveling over and under the wing. If the airplane moves fast enough, the force of this upward lift is able to overcome the force of gravity, and the plane is able to take off and fly.